1. Field of the Invention
The invention is directed to an improved metering system and to a method for operating the metering system, for metering a treatment liquid into an exhaust gas from an internal combustion engine.
2. Description of the Prior Art
To reduce the nitrogen oxides contained in an exhaust gas of an internal combustion engine, for those engines that are operated with air excess, a method for selective catalytic reduction has proved advantageous. In this method, the nitrogen oxides are converted together with ammonia into nitrogen and water in a selective catalytic converter. The reducing agent, required for the catalytic conversion of the nitrogen oxides, is carried along instead of ammonia in the vehicle, in the form of an aqueous urea solution from which the ammonia can be released by hydrolysis of the urea solution in whatever amount is needed for the conversion. It is known to inject the urea solution into an exhaust manifold downstream of a mixing chamber, by means of an aerosol formation that occurs in the mixing chamber. The alternative of metering the urea solution directly into the exhaust gas without air reinforcement is also known. In each case, it is problematic that depending on the urea concentration, there is the risk that the aqueous urea solution will freeze at certain temperatures. The expansion behavior of the urea solution is similar to that of water. If the freezing urea solution is unable to expand, impermissibly high pressures can occur in the interior of components that are filled with the urea solution, and this can lead to the destruction of the components. To avoid this, it is known to provide walls that at least partially yield under pressure in the work chamber of a metering valve, or to provide elements there that can compensate reversibly for a pressure in the work chamber that occurs when ice forms. It is also known to use metering pumps instead of metering valves for metering the urea solution.